Feb 7, 2013

Weak Signal
Audio 160m, a new project

When searching for progression,
sometimes there is a need to look at matters from a totally different
perspective.

In our
efforts to dig deeper in the band-noise, opening up another layer,
making new CW QSO’s possible, we decided to do a different
investigation and perform some tests. This project does not touch on
antennas or any RF/IF hardware. Instead, those steps from the detector
to the 'decoding by human brain'. From your AF stage, via any
transducer, passing ear-wax to that final conversion into gray matter
signals.

Why discuss
the use of headphones?

There's several potential topics to be discussed along this extended
audio path. At some point we need to look at headphones, and perhaps
pick one or more relevant types who might serve during future tests.
Questions arise; are there relevant differences in headphones when it
comes to making that extra QSO? What other parameters exist next
to the obvious ‘wearing comfort’? Is there a single winning
model or do you individually need to test-find a match for the
operator?

List of
headphones under test

No
effort was made to add any specific commercial popular model or
brand. We just grabbed as many HAM-Radio, Hi-Fi, DJ, Advanced Gaming
and/or your typical MP3 headphones as we could get. All full-range
stereo models. Some equipped with in-line volume control(s).

We
started with more than 20 models, slowly narrowing it down to about a handfull which offered small advantages in sudden areas.

Just a small bunch of the tested headphones

Comparison test
conditions

The
sound source was 160m recordings from CQWW-CW- 2012, using the
Microtelecom Perseus. Filter taps were set to 32, as this enhances
weak signal detection and avoids typical digital harshness which is common on some DSP's . During
the first weeks the internal sound card of the laptop was tried, evolving to
external DAC units, different external (Hi-Fi) amplifiers, finally
settling for the premium quality Objective 2 Headphone amplifier.
The O2 is an open source amplifier designed by NwAvGuy, with emphasis
on benchmark performance and low cost.

The listening
test

From
Early December till mid-January we spent two evenings per week
listening to many different, but carefully selected time segments of
the contest recordings, offering:

Weak
signal alone in the band noise

Extreme
weak signals in/out of the band noise being extremely difficult to
read, let alone decode for a valid QSO. Sometimes under lots of QRN
and/or slow QSB. Listening for a longer duration to any CQ’ing
station being called by far-far-away-dx. Sometimes we would eaves-drop on a CQ-íng big-gun, switching his 4SQ 180 deg. back and forth.

Weak
signal close to a loud signal

This
is also a dynamic range test; how does it (and your hearing) deal
with weak signals in an environment of the complexity of loud sounds
and quick volume changes.

Our
main goal was to distinguish between:

How
easy/difficult is it to pick and
decode a weak CW signal?

Which is offering you a betterreading resolution while
tuning the band?

What
other audible effects, good or bad are noticed?

It was
not about the offered sound quality as such. This was about
making an extra QSO Y/N, next to being able to listen over prolonged
periods of time, either comfortably scanning the band or monitoring
QSO after QSO of some running Big-Gun.

Surely
we were subject to the typical learning/recognition process when
repeating sound clips, listening level differences and the inevitable
‘sound memory effect’ when doing endless A/B/X comparisons. But
along the way several headphone models started to show their merits
in one or more areas. This was not about doing a headphone shootout,
but achieving our goal by learning from all aspects.

Noise
cancellation headphones

A
few years ago we did several tests, using different
in-/on-/around-ear active&passive NC models, both at home and our
club station PI4TUE. During 48hrs contests with many operators
available, we received a lot of feedback. At best the NC models
would offer some cancellation of the high environmental noise at low
frequencies. Rumble and fan-noise from > 20 network servers in the
contest room was attenuated to some extent and did offer some relief.
But the active models showed other nasty side effects like
additional internal amp noise, sound distortion and dynamic range
issues. The passive versions, with ear surrounding gel-pads on the
other hand, performed outstanding. Today, 3 operators still use these
at home and in contests. Only the passive noise cancellation gel-pad
version headphones have been used in this test.

Test
results

High
sensitivity HP reveals internal AF amplifier noise &hum

Every
hum, noise, pitched distortions (audio birdies) of the AF amplifier
are being heard. And this effect becomes much
worse going to premium and high-end range models. The more expensive
the headphone the higher sensitivity they usually offer. You need a fairly
loud source input volume to overcome this amplifier QRN.

AF
amp internal noise/hum level must be very low

That
is why we quickly went to search for the best available headphone
amplification ham operator budget can afford. You can build your own
low-noise headphone (distribution) amp, but don’t take it too far;
read the chapter at ‘Designer Components’
From my past years at service and design of consumer audio Hi-Fi
equipment, I recognize these statements all too well.

At
future test we will show AF spectrum outputs of various HF
transceivers.

We
are subject to back ground noise in daily life and have become used
to it. The fixed level of AF amplifier noise in the background
creates an unconscious distraction. This became very apparent when
using the in-line volume controls; making them less sensitive
effectively pushed the AF noise level below the hearing threshold.
You only recognize this benefit when you free yourself from the
distracting noise. However, now offering much higher impedance to
your amps output might affect its performance in other areas. Again
a reason to switch to a quiet amplifier, also capable of handling a
wide impedance range.

Again,
premium headphones reveal them pretty loud. Today, many on- and
in-ear models are ‘tuned for extra bass’. Next to your CW
passband filter settings, additional high pass filtering is a
possible solution. But not used during this test.

Upper
range of audio spectrum HP does not contribute to readability

Obvious
and confirmed while using some experimental settings of filtering,
sound equalization etc. A dedicated lowpass filter is under design,
but never used in this test.

Open
system on HP allow for distraction of environmental noise, decreasing
focus over time

No
problem if your shack is a remote, dead quiet man’s cave. Whatever
your preference, you need at least some closed ear-protection, so you
can concentrate on the job. We find open systems only usable for what
they are intended for; playing music.

Premium
High Fidelity HP offer an aggressive time envelope on
noise/impulses/key-clicks or any radio communication sound. This is
much fatiguing.

Whatever
the great detailed sound and high dynamics offered by the premium to
high-end quality headphones, they have a serious downside. They
closely follow every quick rising signal and reproduce a ‘truth’
which becomes quickly annoying. Interesting during the first minute,
but soon becoming unpleasant for your hearing. It’s like listening
with AGC OFF, times 100. Even clear band noise slowly becomes
fatiguing for your hearing. Let alone any nearby key-clicks, pops
etc. For those using any antenna pre-amplification, be warned; it’s
like sticking a needle in your ear every time the neighbor flicks a
light switch. The overall result is a rough, harsh sound experience.
Despite the much more detailed sound, there was never an advantage
while trying to detect/decode CW weak signals.

For
recognizing and decoding weak CW signals, the low-end or cheaper
models performed equally well, less the harsh and quickly fatiguing
sound experience. A cheap hamradio boom-mic headset was preferred
over its premium competitor. I recall that exact same experience
when I bought the high-end model after having used the former for
several years. But the larger, much more comfortable ear pads and
sturdy mic-boom did make a valid difference. I only use it when making a
phone QSO on 50MHz.

Mind you;
the time envelope of CW signals is relatively slow. And their sound
environment (band noise, off-pitched CW signals)is such different,
that we do not need any additional sound resolution to distinguish
them better.

Premium
headphone models just offer too much; less seems better.

Communication
drivers/cheaper HP models seem to have a mechanical threshold. This
can offer less detail at very low listening volume, perhaps also
masking weak signals by amplifier noise

Although
we were splitting hairs, a few low end models showed this effect. It
became more apparent while using noisy amplifiers and/or connected to some of our HF
rigs. Turning up the volume cleared the effect.

Louder
is NOT better

Turning
up the volume for completing that difficult QSO is not enhancing your
hearing ability.

Interesting
remark: “another important effect of loudness perception is that
not every sound that is above your threshold of hearing can be heard.
If a sound is given, while a near louder one is also present (within
the auditory filter) then it is possible you do not perceive the
lower one. This process seems intuitive, but on the psychoacoustic
and cognitive levels it becomes very complex. The term for this
process is masking.” In short: your hearing ability has a minimal
bandwidth and a varying curve.

Guess how you are going to compensate for that lack of hearing ability?

Right; you will turn up the volume even more....

And
exposure to loud listening volume slowly might activate your
‘Acoustic Reflex’, the attenuation reflex in your middle ear. And again you compensate by reaching to that volume knob.

Although
turning up your listening volume seems effective sometimes, it
certainly has more to do with your current focus and perhaps
the continuous distraction from environmental noise.

Closed
HP systems with passive noise cancellation allow for high focus at an
average lower listening volume. Separation of weak signals is easier
at lower audio levels here.

The
ear surrounding gel pads model from the professional aviation
headsets offer clear, isolated reception of weak signals in noisy
environments. They avoid listening fatigue in lengthy contests. The
gel pad aviation model was friendly for the ears, even after many
hours of contest use. But one of our clubstation operators perceived the ‘ear
sealing effect’ as uncomfortable. Another instead, uses a generic
hearing protector around his in-ear headphone, like drummers do in
their studio.

And
some final remarks

'Clean' TX signals, with less sidebands/key-clicks, resulting at AF level in more approaching a true sinus character,
are easier to decode as a weak signal

Sound
timbre and tonal coloring are a matter of personal
choice, as is wearing
comfort

Passive/active
sound moderation like limiting and/or low-pass filtering may have a
pronounced effect, but are to be proven as we have not used these in
this test.